I doubt it's cash upfront, they'll probably just reimburse you upon your return to their casino for some number of nights. Read the fine print, and note any restrictions very carefully.
Here's an example:
Book by August 31, 2008. Flyback Certificate may only be redeemed for your return trip to MGM Grand and is subject to terms and conditions noted on the certificate. The return trip (hotel and air) must include two or more nights at MGM Grand and must be booked through travel agency designated on certificate. Flyback certificate will be issued to qualified guestss upon check-in. The $25 activity credit is valid for dining or entertainment. Dining credit is not valid at In-room dining, Joel Robuchon, L'Atelier de Joel Robuchon, Emeril's, Rainforest Cafe, Stage Deli, and Starbucks. Entertainment credit is valid for KA by Cirque du Soleil or Crazy Horse Paris and may only be redeemed after check-in. 15% off Retail shopping is valid for MGM Grand & Co. and Xclusiv. Guests must be at least 18 years of age to utilize spa or receive spa services. Complimentary cocktails are valid at MGM Grand Pools. Guest must charge inclusions to room folio and will be credited upon check-out. Inclusions are per package, per stay and have no cash value. Rates are subject to change and restricted dates apply. Suites and suite bed type are based upon availability upon check-in. We are unable to guarantee specific room types and locations, however a sincere effort will be made to accommodate your needs. Not available for existing reservations. This promotion is non-transferable, subject to availability, may not be combined with any other MGM Grand promotion and is not available to groups or persons attending meetings or conventions or over certain blacked-out dates. All prices are subject to tax and/or handling fee and based on double occupancy on a per-stay basis. MGM Grand reserves the right to cancel or modify this promotion at its discretion and without prior notice.
I'd be particularly careful about their booking agency, since that gives them a lot of control over price of the flight. They also have control over dates, which can stick you with an expensive room.
But I'd like to hear from others as to how egregious they are; even if they have the power to screw you on the deal, they may value your business enough to be nice.
